protistas

Protists are a diverse group of eukaryotic microorganisms that do not fit neatly into the traditional kingdoms of animals, plants, or fungi. The term originated in the 19th century as a catch-all for primitive life forms and was proposed by Ernst Haeckel as a separate kingdom. In contemporary biology, Protists are not considered a natural, unified group; many protists belong to several distinct eukaryotic lineages. Molecular phylogenetics has shown that the organisms once placed in Protista are scattered across multiple major supergroups, such as Excavata, SAR, Archaeplastida, and Unikonta. As a result, modern classifications often avoid a formal Protista kingdom and instead classify protist-like organisms within several clades or kingdoms.

Protists are highly diverse in form and metabolism. They are predominantly unicellular, but some are colonial or truly multicellular. They range from amoeboid and flagellate forms to single-celled algae and complex ciliates. Metabolism also varies: photosynthetic protists include many algae, while others are heterotrophic, feeding by ingesting or absorbing organic matter, and some are parasites.

Ecologically, protists play key roles as primary producers, consumers, decomposers, and pathogens. They inhabit a wide range of environments, especially aquatic habitats, soils, and the bodies of animals and plants. Reproduction occurs by asexual processes such as binary fission and budding, and many protists have sexual life stages that involve meiosis and genetic exchange.

Because Protists are not a monophyletic group, their usage is increasingly restricted to historical or convenience contexts, with modern taxonomy preferring more precise clade-based classifications.